Neural networks are formed from multiple neurons which communicate with each other via synapses in order to deliver or process information via chemical and electrical signaling. For a chemical synapse, a complex sequence of events causes the neuron to fire. However, regardless of the various physical mechanisms of the communication at chemical synapses, the effect of synaptic input can be classed as either excitatory (raising membrane potential) or inhibitory (lowering membrane potential). Neurons also communicate via a second type of synapse called gap junctions or electrical synapses which fulfil a role similar to chemical synapses. Gap junctions, unlike chemical synapses, introduce no time delay in electrical signalling from the pre-synaptic neuron to the post-synaptic neuron.
Central pattern generators (CPG) are neural networks that are responsible for coordinating motor activity in animals and humans and endogenously generating the rhythms that sustain life. Central pattern generators typically serve multiple functions ranging from roles which are central to movement, breathing, swimming, swallowing, rhythm generation and other oscillatory functions, transient or permanent. Central pattern generators are found in all animals from invertebrates such as leeches or gastropods to the brain stem of mammals.